Modernize android build and sample (#6229)

Android build was dated, using the Android.mk approach. Current
project configuration on Android encourages the usage of CMake, so we
are updating the android project as an example on how to use either the
Java/Kotlin generate code or the native C++ one.

android/app/src/main/cpp/CMakeLists.txt

@@ -0,0 +1,54 @@
+# For more information about using CMake with Android Studio, read the
+# documentation: https://d.android.com/studio/projects/add-native-code.html
+
+# Sets the minimum version of CMake required to build the native library.
+
+cmake_minimum_required(VERSION 3.4.1)
+
+# Creates and names a library, sets it as either STATIC
+# or SHARED, and provides the relative paths to its source code.
+# You can define multiple libraries, and CMake builds them for you.
+# Gradle automatically packages shared libraries with your APK.
+
+include_directories(${FLATBUFFERS_SRC}/include)
+
+add_subdirectory(flatbuffers)
+
+FILE(GLOB Generated_SRCS generated/*.h)
+
+add_library( # Sets the name of the library.
+             native-lib
+
+             # Sets the library as a shared library.
+             SHARED
+
+             # Provides a relative path to your source file(s).
+             animals.cpp
+             ${Generated_SRCS}
+
+)
+
+# Searches for a specified prebuilt library and stores the path as a
+# variable. Because CMake includes system libraries in the search path by
+# default, you only need to specify the name of the public NDK library
+# you want to add. CMake verifies that the library exists before
+# completing its build.
+
+find_library( # Sets the name of the path variable.
+              log-lib
+
+              # Specifies the name of the NDK library that
+              # you want CMake to locate.
+              log )
+
+# Specifies libraries CMake should link to your target library. You
+# can link multiple libraries, such as libraries you define in this
+# build script, prebuilt third-party libraries, or system libraries.
+
+target_link_libraries( # Specifies the target library.
+                       native-lib
+                       flatbuffers
+                       flatbuffers_tests
+                       # Links the target library to the log library
+                       # included in the NDK.
+                       ${log-lib} )

android/app/src/main/cpp/animals.cpp

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2014 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <jni.h>
+#include <string>
+#include <search.h>
+#include "generated/animal_generated.h"
+
+using namespace com::fbs::app;
+using namespace flatbuffers;
+
+extern "C" JNIEXPORT jbyteArray JNICALL Java_com_flatbuffers_app_MainActivity_createAnimalFromJNI(
+        JNIEnv* env,
+        jobject /* this */) {
+    // create a new animal flatbuffers
+    auto fb = FlatBufferBuilder(1024);
+    auto tiger = CreateAnimalDirect(fb, "Tiger", "Roar", 300);
+    fb.Finish(tiger);
+
+    // copies it to a Java byte array.
+    auto buf = reinterpret_cast<jbyte*>(fb.GetBufferPointer());
+    int size = fb.GetSize();
+    auto ret = env->NewByteArray(size);
+    env->SetByteArrayRegion (ret, 0, fb.GetSize(), buf);
+  return ret;
+}

android/app/src/main/cpp/flatbuffers/CMakeLists.txt

@@ -0,0 +1,59 @@
+# For more information about using CMake with Android Studio, read the
+# documentation: https://d.android.com/studio/projects/add-native-code.html
+
+# Sets the minimum version of CMake required to build the native library.
+
+cmake_minimum_required(VERSION 3.4.1)
+
+include_directories(${FLATBUFFERS_SRC}/include)
+
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11 -fexceptions -Wall -DFLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE")
+
+# Certain platforms such as ARM do not use signed chars by default
+# which causes issues with certain bounds checks.
+set(CMAKE_CXX_FLAGS
+   "${CMAKE_CXX_FLAGS} -fsigned-char")
+
+set(FlatBuffers_Library_SRCS
+        ${FLATBUFFERS_SRC}/include/flatbuffers/base.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/flatbuffers.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/hash.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/idl.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/util.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/reflection.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/reflection_generated.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/stl_emulation.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/flexbuffers.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/registry.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/minireflect.h
+        ${FLATBUFFERS_SRC}/src/idl_parser.cpp
+        ${FLATBUFFERS_SRC}/src/idl_gen_text.cpp
+        ${FLATBUFFERS_SRC}/src/reflection.cpp
+        ${FLATBUFFERS_SRC}/src/util.cpp
+        ${FLATBUFFERS_SRC}/src/idl_gen_fbs.cpp
+        ${FLATBUFFERS_SRC}/src/code_generators.cpp
+        )
+
+set(FlatBuffers_Test_SRCS
+        ${FLATBUFFERS_SRC}/tests/test.cpp
+        ${FLATBUFFERS_SRC}/tests/test_assert.h
+        ${FLATBUFFERS_SRC}/tests/test_builder.h
+        ${FLATBUFFERS_SRC}/tests/test_assert.cpp
+        ${FLATBUFFERS_SRC}/tests/test_builder.cpp
+        ${FLATBUFFERS_SRC}/tests/native_type_test_impl.h
+        ${FLATBUFFERS_SRC}/tests/native_type_test_impl.cpp
+)
+
+add_library( # Sets the name of the library.
+             flatbuffers
+
+             ${FlatBuffers_Library_SRCS}
+             ${FlatBuffers_Test_SRCS}
+             ${Generated_SRCS}
+)
+
+add_library( # Sets the name of the library.
+             flatbuffers_tests
+
+             ${FlatBuffers_Test_SRCS}
+)

android/app/src/main/cpp/generated/animal_generated.h

@@ -0,0 +1,128 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+
+#ifndef FLATBUFFERS_GENERATED_ANIMAL_COM_FBS_APP_H_
+#define FLATBUFFERS_GENERATED_ANIMAL_COM_FBS_APP_H_
+
+#include "flatbuffers/flatbuffers.h"
+
+namespace com {
+namespace fbs {
+namespace app {
+
+struct Animal;
+struct AnimalBuilder;
+
+struct Animal FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
+  typedef AnimalBuilder Builder;
+  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {
+    VT_NAME = 4,
+    VT_SOUND = 6,
+    VT_WEIGHT = 8
+  };
+  const flatbuffers::String *name() const {
+    return GetPointer<const flatbuffers::String *>(VT_NAME);
+  }
+  const flatbuffers::String *sound() const {
+    return GetPointer<const flatbuffers::String *>(VT_SOUND);
+  }
+  uint16_t weight() const {
+    return GetField<uint16_t>(VT_WEIGHT, 0);
+  }
+  bool Verify(flatbuffers::Verifier &verifier) const {
+    return VerifyTableStart(verifier) &&
+           VerifyOffset(verifier, VT_NAME) &&
+           verifier.VerifyString(name()) &&
+           VerifyOffset(verifier, VT_SOUND) &&
+           verifier.VerifyString(sound()) &&
+           VerifyField<uint16_t>(verifier, VT_WEIGHT) &&
+           verifier.EndTable();
+  }
+};
+
+struct AnimalBuilder {
+  typedef Animal Table;
+  flatbuffers::FlatBufferBuilder &fbb_;
+  flatbuffers::uoffset_t start_;
+  void add_name(flatbuffers::Offset<flatbuffers::String> name) {
+    fbb_.AddOffset(Animal::VT_NAME, name);
+  }
+  void add_sound(flatbuffers::Offset<flatbuffers::String> sound) {
+    fbb_.AddOffset(Animal::VT_SOUND, sound);
+  }
+  void add_weight(uint16_t weight) {
+    fbb_.AddElement<uint16_t>(Animal::VT_WEIGHT, weight, 0);
+  }
+  explicit AnimalBuilder(flatbuffers::FlatBufferBuilder &_fbb)
+        : fbb_(_fbb) {
+    start_ = fbb_.StartTable();
+  }
+  AnimalBuilder &operator=(const AnimalBuilder &);
+  flatbuffers::Offset<Animal> Finish() {
+    const auto end = fbb_.EndTable(start_);
+    auto o = flatbuffers::Offset<Animal>(end);
+    return o;
+  }
+};
+
+inline flatbuffers::Offset<Animal> CreateAnimal(
+    flatbuffers::FlatBufferBuilder &_fbb,
+    flatbuffers::Offset<flatbuffers::String> name = 0,
+    flatbuffers::Offset<flatbuffers::String> sound = 0,
+    uint16_t weight = 0) {
+  AnimalBuilder builder_(_fbb);
+  builder_.add_sound(sound);
+  builder_.add_name(name);
+  builder_.add_weight(weight);
+  return builder_.Finish();
+}
+
+inline flatbuffers::Offset<Animal> CreateAnimalDirect(
+    flatbuffers::FlatBufferBuilder &_fbb,
+    const char *name = nullptr,
+    const char *sound = nullptr,
+    uint16_t weight = 0) {
+  auto name__ = name ? _fbb.CreateString(name) : 0;
+  auto sound__ = sound ? _fbb.CreateString(sound) : 0;
+  return com::fbs::app::CreateAnimal(
+      _fbb,
+      name__,
+      sound__,
+      weight);
+}
+
+inline const com::fbs::app::Animal *GetAnimal(const void *buf) {
+  return flatbuffers::GetRoot<com::fbs::app::Animal>(buf);
+}
+
+inline const com::fbs::app::Animal *GetSizePrefixedAnimal(const void *buf) {
+  return flatbuffers::GetSizePrefixedRoot<com::fbs::app::Animal>(buf);
+}
+
+inline bool VerifyAnimalBuffer(
+    flatbuffers::Verifier &verifier) {
+  return verifier.VerifyBuffer<com::fbs::app::Animal>(nullptr);
+}
+
+inline bool VerifySizePrefixedAnimalBuffer(
+    flatbuffers::Verifier &verifier) {
+  return verifier.VerifySizePrefixedBuffer<com::fbs::app::Animal>(nullptr);
+}
+
+inline void FinishAnimalBuffer(
+    flatbuffers::FlatBufferBuilder &fbb,
+    flatbuffers::Offset<com::fbs::app::Animal> root) {
+  fbb.Finish(root);
+}
+
+inline void FinishSizePrefixedAnimalBuffer(
+    flatbuffers::FlatBufferBuilder &fbb,
+    flatbuffers::Offset<com::fbs::app::Animal> root) {
+  fbb.FinishSizePrefixed(root);
+}
+
+}  // namespace app
+}  // namespace fbs
+}  // namespace com
+
+#endif  // FLATBUFFERS_GENERATED_ANIMAL_COM_FBS_APP_H_